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NAME
os - Operating system-specific functions.
DESCRIPTION
The functions in this module are operating system-specific. Careless use of these
functions results in programs that will only run on a specific platform. On the other
hand, with careful use, these functions can be of help in enabling a program to run on
most platforms.
Note:
File operations used to accept filenames containing null characters (integer value zero).
This caused the name to be truncated and in some cases arguments to primitive operations
to be mixed up. Filenames containing null characters inside the filename are now rejected
and will cause primitive file operations to fail.
Also environment variable operations used to accept names and values of environment
variables containing null characters (integer value zero). This caused operations to
silently produce erroneous results. Environment variable names and values containing null
characters inside the name or value are now rejected and will cause environment variable
operations to fail.
DATA TYPES
env_var_name() = nonempty_string()
A string containing valid characters on the specific OS for environment variable
names using file:native_name_encoding() encoding. Note that specifically null
characters (integer value zero) and $= characters are not allowed. However, note
that not all invalid characters necessarily will cause the primitiv operations to
fail, but may instead produce invalid results.
env_var_value() = string()
A string containing valid characters on the specific OS for environment variable
values using file:native_name_encoding() encoding. Note that specifically null
characters (integer value zero) are not allowed. However, note that not all invalid
characters necessarily will cause the primitiv operations to fail, but may instead
produce invalid results.
env_var_name_value() = nonempty_string()
Assuming that environment variables has been correctly set, a strings containing
valid characters on the specific OS for environment variable names and values using
file:native_name_encoding() encoding. The first $= characters appearing in the
string separates environment variable name (on the left) from environment variable
value (on the right).
os_command() = atom() | io_lib:chars()
All characters needs to be valid characters on the specific OS using
file:native_name_encoding() encoding. Note that specifically null characters
(integer value zero) are not allowed. However, note that not all invalid characters
not necessarily will cause os:cmd/1 to fail, but may instead produce invalid
results.
os_command_opts() = #{max_size => integer() >= 0 | infinity}
Options for os:cmd/2
max_size:
The maximum size of the data returned by the os:cmd call. See the os:cmd/2
documentation for more details.
EXPORTS
cmd(Command) -> string()
cmd(Command, Options) -> string()
Types:
Command = os_command()
Options = os_command_opts()
Executes Command in a command shell of the target OS, captures the standard output
of the command, and returns this result as a string.
Warning:
Previous implementation used to allow all characters as long as they were integer
values greater than or equal to zero. This sometimes lead to unwanted results since
null characters (integer value zero) often are interpreted as string termination.
The current implementation rejects these.
Examples:
LsOut = os:cmd("ls"), % on unix platform
DirOut = os:cmd("dir"), % on Win32 platform
Notice that in some cases, standard output of a command when called from another
program (for example, os:cmd/1) can differ, compared with the standard output of
the command when called directly from an OS command shell.
os:cmd/2 was added in kernel-5.5 (OTP-20.2.1). It makes it possible to pass an
options map as the second argument in order to control the behaviour of os:cmd. The
possible options are:
max_size:
The maximum size of the data returned by the os:cmd call. This option is a
safety feature that should be used when the command executed can return a very
large, possibly infinite, result.
> os:cmd("cat /dev/zero", #{ max_size => 20 }).
[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
find_executable(Name) -> Filename | false
find_executable(Name, Path) -> Filename | false
Types:
Name = Path = Filename = string()
These two functions look up an executable program, with the specified name and a
search path, in the same way as the underlying OS. find_executable/1 uses the
current execution path (that is, the environment variable PATH on Unix and
Windows).
Path, if specified, is to conform to the syntax of execution paths on the OS.
Returns the absolute filename of the executable program Name, or false if the
program is not found.
getenv() -> [env_var_name_value()]
Returns a list of all environment variables. Each environment variable is expressed
as a single string on the format "VarName=Value", where VarName is the name of the
variable and Value its value.
If Unicode filename encoding is in effect (see the erl manual page), the strings
can contain characters with codepoints > 255.
getenv(VarName) -> Value | false
Types:
VarName = env_var_name()
Value = env_var_value()
Returns the Value of the environment variable VarName, or false if the environment
variable is undefined.
If Unicode filename encoding is in effect (see the erl manual page), the strings
VarName and Value can contain characters with codepoints > 255.
getenv(VarName, DefaultValue) -> Value
Types:
VarName = env_var_name()
DefaultValue = Value = env_var_value()
Returns the Value of the environment variable VarName, or DefaultValue if the
environment variable is undefined.
If Unicode filename encoding is in effect (see the erl manual page), the strings
VarName and Value can contain characters with codepoints > 255.
getpid() -> Value
Types:
Value = string()
Returns the process identifier of the current Erlang emulator in the format most
commonly used by the OS environment. Returns Value as a string containing the
(usually) numerical identifier for a process. On Unix, this is typically the return
value of the getpid() system call. On Windows, the process id as returned by the
GetCurrentProcessId() system call is used.
putenv(VarName, Value) -> true
Types:
VarName = env_var_name()
Value = env_var_value()
Sets a new Value for environment variable VarName.
If Unicode filename encoding is in effect (see the erl manual page), the strings
VarName and Value can contain characters with codepoints > 255.
On Unix platforms, the environment is set using UTF-8 encoding if Unicode filename
translation is in effect. On Windows, the environment is set using wide character
interfaces.
Note:
VarName is not allowed to contain an $= character. Previous implementations used to
just let the $= character through which silently caused erroneous results. Current
implementation will instead throw a badarg exception.
set_signal(Signal, Option) -> ok
Types:
Signal =
sighup | sigquit | sigabrt | sigalrm | sigterm | sigusr1 |
sigusr2 | sigchld | sigstop | sigtstp
Option = default | handle | ignore
Enables or disables OS signals.
Each signal my be set to one of the following options:
ignore:
This signal will be ignored.
default:
This signal will use the default signal handler for the operating system.
handle:
This signal will notify erl_signal_server when it is received by the Erlang
runtime system.
system_time() -> integer()
Returns the current OS system time in native time unit.
Note:
This time is not a monotonically increasing time.
system_time(Unit) -> integer()
Types:
Unit = erlang:time_unit()
Returns the current OS system time converted into the Unit passed as argument.
Calling os:system_time(Unit) is equivalent to
erlang:convert_time_unit(os:system_time(), native, Unit).
Note:
This time is not a monotonically increasing time.
timestamp() -> Timestamp
Types:
Timestamp = erlang:timestamp()
Timestamp = {MegaSecs, Secs, MicroSecs}
Returns the current OS system time in the same format as erlang:timestamp/0. The
tuple can be used together with function calendar:now_to_universal_time/1 or
calendar:now_to_local_time/1 to get calendar time. Using the calendar time,
together with the MicroSecs part of the return tuple from this function, allows you
to log time stamps in high resolution and consistent with the time in the rest of
the OS.
Example of code formatting a string in format "DD Mon YYYY HH:MM:SS.mmmmmm", where
DD is the day of month, Mon is the textual month name, YYYY is the year, HH:MM:SS
is the time, and mmmmmm is the microseconds in six positions:
-module(print_time).
-export([format_utc_timestamp/0]).
format_utc_timestamp() ->
TS = {_,_,Micro} = os:timestamp(),
{{Year,Month,Day},{Hour,Minute,Second}} =
calendar:now_to_universal_time(TS),
Mstr = element(Month,{"Jan","Feb","Mar","Apr","May","Jun","Jul",
"Aug","Sep","Oct","Nov","Dec"}),
io_lib:format("~2w ~s ~4w ~2w:~2..0w:~2..0w.~6..0w",
[Day,Mstr,Year,Hour,Minute,Second,Micro]).
This module can be used as follows:
1> io:format("~s~n",[print_time:format_utc_timestamp()]).
29 Apr 2009 9:55:30.051711
OS system time can also be retreived by system_time/0 and system_time/1.
perf_counter() -> Counter
Types:
Counter = integer()
Returns the current performance counter value in perf_counter time unit. This is a
highly optimized call that might not be traceable.
perf_counter(Unit) -> integer()
Types:
Unit = erlang:time_unit()
Returns a performance counter that can be used as a very fast and high resolution
timestamp. This counter is read directly from the hardware or operating system with
the same guarantees. This means that two consecutive calls to the function are not
guaranteed to be monotonic, though it most likely will be. The performance counter
will be converted to the resolution passed as an argument.
1> T1 = os:perf_counter(1000),receive after 10000 -> ok end,T2 = os:perf_counter(1000).
176525861
2> T2 - T1.
10004
type() -> {Osfamily, Osname}
Types:
Osfamily = unix | win32
Osname = atom()
Returns the Osfamily and, in some cases, the Osname of the current OS.
On Unix, Osname has the same value as uname -s returns, but in lower case. For
example, on Solaris 1 and 2, it is sunos.
On Windows, Osname is nt.
Note:
Think twice before using this function. Use module filename if you want to inspect
or build filenames in a portable way. Avoid matching on atom Osname.
unsetenv(VarName) -> true
Types:
VarName = env_var_name()
Deletes the environment variable VarName.
If Unicode filename encoding is in effect (see the erl manual page), the string
VarName can contain characters with codepoints > 255.
version() -> VersionString | {Major, Minor, Release}
Types:
VersionString = string()
Major = Minor = Release = integer() >= 0
Returns the OS version. On most systems, this function returns a tuple, but a
string is returned instead if the system has versions that cannot be expressed as
three numbers.
Note:
Think twice before using this function. If you still need to use it, always call
os:type() first.